1. Field of the Invention
The present invention relates to a method of manufacturing a glass fiber preform for single-mode optical fibers.
2. Background of the Prior Art
There has been well-known a method of manufacturing a glass fiber preform for single-mode fibers. In this method, a glass raw material is subject to a flame hydrolysis reaction, or the like, to form a porous glass body composed of a core and a cladding. The resultant porous glass body is dehydrated and sintered so as to obtain a collapsed intermediate glass rod. Porous quartz glass is further formed by the above-mentioned method at the outside of the obtained intermediate glass rod composed of the core and the cladding. The thus obtained composition is sintered or dehydrated and sintered to thereby obtain a required cladding thickness.
In the above-mentioned conventional method, the dehydration effected in producing the intermediate glass rod is very important to keep the transmission loss low when the glass fiber preform is used for making optical fibers. Therefore, a method is employed in which a gas of the chlorine group is contained in an atmosphere as a dehydrating agent. However, chlorine remains in the sintered intermediate body to make the refractive index of the cladding a little high so that a step in the refractive index is undesirably produced between the intermediate glass rod and the glass to be formed at the outside of the intermediate glass rod. As a result, the effective cut-off wavelength for a non-transmitted high order mode (hereinafter referred to simply as "cut off wavelength") is increased in the transmission characteristics of the optical fibers. Therefore, a method is employed in which dehydration treatment is performed by using a gas in the chlorine group as a dehydrating agent when the porous glass outside an intermediate glass body is collapsed similarly as in the case of the intermediate body (U.S. Pat. No. 4,486,212). In this method, as shown in FIG. 1, a porous glass body 2' is subject to heat treatment in an atmosphere containing a gas of the chlorine group. A heater 8 surrounds a furnace central tube 9. This method has disadvantages in that it is difficult to homogeneously incorporate the chlorine group within the porous glass body 2' because it takes a long time for the chlorine group gas to diffuse and permeate into the inside of the porous glass body 2' to react with the glass body. A further disadvantage is that if the concentration of the chlorine group gas is increased to shorten the treatment time, an excessive portion of the chlorine group gas is apt to remain as bubbles in the step of collapsing the porous glass body 2'.
As described above, in the method disclosed in U.S. Pat. No. 4,486,212, there have been such problems that when a complex of a porous glass body and an intermediate glass rod is inserted into an atmosphere containing a gas of a chlorine group in the inside of a furnace so as to be subject to treatment such as heating and dehydration (or the addition of chlorine), it takes a long time for performing the treatment homogeneously in the inside of the complex. Furthermore, if the concentration of the chlorine gas group is made higher to shorten the treatment time, an excessive part of the chlorine gas remains so that bubbles may remain in the inside of the collapsed body in the sintering treatment.